Telecommunication systems, cable television systems, and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers comprise thin strands of glass capable of transmitting a signal over long distances with very little loss. The optical signals have at least one characteristic modulated to encode audio, video, textual, real time, non-real time, and/or other suitable data.
Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity, which can be tens to several hundred Gigabits per second (Gb/s). In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is increased as a multiple of the number of wavelengths, or channels, in each fiber.
The maximum distance that optical signals can be transmitted in a WDM or other optical network is limited, in part, by dispersion and nonlinear effects. For example, chromatic dispersion widens optical pulses transmitting over an optical link, creating in intersymbol interference. Chromatic dispersion results from the frequency dependence of the refractive index of silica as well as waveguide contributions to the effective refractive index. Furthermore, optical signals traveling at high peak power levels, such as high-bit rate WDM signals (e.g., 40, 80, or 160 Gb/s), are subject to nonlinear distortions not discernable at low fiber input power. As bit rates increase, the optical signal-to-noise (OSNR) becomes critical, so high fiber input power is used in an attempt to satisfy the OSNR requirement. However, higher fiber input power leads to signal degradation from fiber nonlinearities, such as self-phase modulation (SPM).
To transmit high-bit rate signals over long distances, for example 160 Gb/s over 1500 km, optical networks typically include a number of dispersion compensating modules spaced (≈100 km) along each optical link which has an opposite dispersion effect for a specific range of wavelengths to most transmission optical fibers. Accurately managing these dispersion compensating modules typically includes measuring the dispersion value in advance at each repeater node to prevent an error in dispersion compensation, employing a plurality of dispersion compensating modules with a variety of fiber spans and types, and re-adjusting or re-designing the dispersion compensating modules based on the dispersion compensating value.